The present invention relates to a method and to a system allowing to increase the cracking corrosion resistance of a wire under stress used for manufacturing armoured flexible pipes intended for petroleum reservoirs development.
It is well-known that certain metals are sensitive to cracking corrosion when they are exposed to certain aqueous corrosive environments, notably aqueous environments containing hydrogen sulfide, and simultaneously subjected to tensile stresses, their sensitivity to cracking corrosion being all the higher as the plastic deformation under tensile stress is high. Stress cracking in a corrosive environment generally develops from the surface.
Stress cracking resistance in an environment containing hydrogen sulfide (SSC: Sulfide Stress Cracking) is a determining property for steels intended for flexible oil pipes reinforcement. In fact, it conditions the nature of the steels that can be used, both in terms of composition and of state of treatment.
The use of low-alloy tempered and hardened steel wires is now conventional for the tension layers of flexible pipes manufactured in the industry. This metallurgy is the best compromise between mechanical strength and SSC resistance.
Documents FR-1,426,113, GB-1,054,979 and DE-1,227,491 notably describe processes for improving the resistance of materials to corrosion, wherein these metals are subjected to cold mechanical surface treatments. The improvement is explained by the fact that the surface is placed under compression, which is opposed to the external tensile stresses and thus reduces the risk of incipient cracking.
However, compression due to the surface treatments proposed in the prior art is accompanied by substantial strain hardening of the surface. This strain hardening reflects a high dislocation density, which makes the metal more sensitive to stress cracking in a corrosive environment. Furthermore, strain hardening through surface treatment according to the prior art damages the surface. In fact, the roughness of the surface treated increases considerably.
Patent FR-2,543,976 proposes a method for placing wires under superficial compression by means of a succession of flexions. However, during manufacture of the flexible pipes, the wires are deformed during the armouring operation which consists in winding said wires around the core consisting of the underlying layers. The most critical part of the wire is the part that has undergone the greatest permanent tensile strain and which is therefore situated on the external face of the flexible pipe. The elongation ratios generated can reach or even exceed 5%, considering the typical dimensions of the flexible pipes and of the wires. Work on this subject has shown that this deformation level could lead to a considerable fall in the SSC resistance: the non-breaking threshold stress becomes markedly lower than the required level, i.e. 90% of the yield strength Rp0.2.
The main object of the invention is to improve, in relation to the prior art, the SSC protection of flexible pipe reinforcements subjected to tensile stresses. The invention therefore proposes brushing the surface of a reinforcing wire after setting it on a flexible pipe.
The invention is basically defined as a method of manufacturing a flexible pipe reinforced with wires, wherein the outer surface of said wires is subjected to a surface treatment by brushing after setting said wires on said flexible pipe.
The invention also relates to a system for manufacturing a flexible pipe reinforced with wires, wherein means intended for surface treatment by brushing of the outer surface of said wires are arranged downstream from the zone where said wires are set in relation to the direction of feed of the flexible pipe.
The stress level that can be obtained depends on the brushing conditions and on the intrinsic characteristics of the material. This compression is due to the plasticization of the surface layers of the metal. However, the nature of the strain hardening is different from that conventionally obtained by sandblasting, machining or shot blasting. In fact, the surface of the brushed parts is less xe2x80x9cbrutalizedxe2x80x9d than with the processes proposed in the prior art, which leads to a low strain hardening degree, to a low mobile dislocation density and to a limited roughness of the surface treated. Brushing thus is an efficient means for limiting fatigue problems and for reducing risks of stress-corrosion cranking.
Furthermore, many parameters (wire exit, brush diameter, rotating speed, bristle diameter, . . .) are available to the user and allow him to obtain, according to the desired use: either a greatly compressed surface but not very altered in depth, or a more penetrating compression associated with lesser deformations and stress levels at the surface.
Treatment of the surface of the wire is advantageously carried out after it has been set on the flexible pipe. Thus, the surface treatment is not altered by the stresses and the strain hardening induced upon setting of the wire on the flexible pipe.